Many types of fibers used for making, for example, cloth, contain dyes or pigments to impart different colors to the fibers. These dyes and pigments typically operate by absorbing some wavelengths of light, while permitting other wavelengths to be reflected. Polymer fibers are a different class of fiber in that they impart color by selectively reflecting some wavelengths of the light and transmitting the other wavelengths. In the past, polymer fibers have used a planar construction of several alternating layers of different polymer materials: the refractive indices and thicknesses of the layers determine the wavelengths that are reflected and those that are transmitted. Fibers of planar layers are, however, difficult to manufacture and are limited to using materials that adhere well to each other.
One use for polymer fibers is in the manufacture of important articles, such as bank notes, securities, permit cards, identification cards (ID cards) or passports. These types of important articles are commonly provided with a number of security measures whose object is to make counterfeiting difficult, if not impossible. Several layers of security features are often used, which are chosen from different technology fields, manufactured by different suppliers, and embodied in different constituting parts of the security article. To break the security document, the counterfeiter would need to obtain all of the implied materials and to get access to all of the required processing technology, which is an extremely difficult task.
The manufacturing of a banknote, for example, requires a special banknote paper (which may contain watermarks, security threads, fibers, planchettes, luminescent particles, windows, foils, decals, coatings, etc.), which are imprinted with particular inks (which may contain security dyes, pigments and further security additives), using dedicated intaglio and other high-security printing equipment. Contrary to commercial printing applications, security printing relies on several different printing techniques combined together on the same document. More recently, banknotes have been provided with security foils and other add-ons, which are applied by dedicated equipment. The materials and the equipment needed to make a banknote are only available from specialized suppliers, and the banknote can only be produced via a high-security printing work by skilled operators.
The security thread is another traditional element of anti-falsification paper for banknotes and other security documents. Embedded security threads are particularly difficult to counterfeit, because a) the thread is made by a specialized vacuum deposition or other highly specialized technique, or is a multilayer film, but b) the thread needs to be incorporated into the printing substrate during the substrate's manufacturing step. This necessary access to two different manufacturing technologies constitutes an obstacle for would-be counterfeiters.
The first-generation security thread for currency was a metal- or a metallized polymer-strip, entirely buried within the currency paper, and authenticate-able either by looking at it in translucency, or by detecting its electric or magnetic properties with the help of a corresponding device. More sophisticated versions of the security thread include window-threads, which are partly buried within the paper and partly lie open at the surface of the paper. This allows for a direct visual authentication of the thread's overt security features at the sites where it appears at the paper surface.
One particular laminated window thread, comprising an optically variable interference coating manufactured by the vacuum deposition of a multi-layer interference stack onto a carrier plastic sheet, and subsequently protected by laminating a second plastic sheet over it. Currency paper containing such optical threads, which change color depending on the viewing angle, is currently available.
Vacuum coating is an expensive process, with a concomitantly high price for such threads. There is a need, therefore, to produce threads less expensively.